Process for the vulcanization of rubber



United States Patent 3,111,564 PROCESS FOR THE VULCANIZATEGN 6F RUBBER Friedrich Loher, Leverknsen-Bayerwerh, Ernst Boos,

Cologne-Flittard, and Hans Pohle, Leverhusen, Germany, assignors to Farbenfabriken Bayer Ahtiengesellschaft, Leverkusen, Germany, a corporation of Germany No Drawing Filed Get. 28, 1959, Ser. No. 849,136 Claims priority, application Germany Get. 31, 1953 4 Claims. (Cl. 260-795) This invention relates to the vulcanization of rubber and to compounds useful in such a vulcanization.

The properties of a vulcanizate of natural or synthetic rubber are influenced by the nature of the vulcanizing agent as well as by other factors. When elementary sulphur is used as vulcanizing agent, it is known that ageing phenomena are observed which are attributed for example to acid formation and which deleteriously affect the properties of the vuloanizates. Such sulphur vulcanizates show a strong tendency to take up oxygen with the ageing. Consequently, an attempt has already been made to replace elementary sulphur in rubber mixtures wholly or partially by thiuram polysulphides or amine disulphides in order to improve the vulcanization properties and the ageing of such vulcanizates.

It has now been found that when vulcanizing natural or synthetic rubber, particularly good vulcanizates are obtained if the vulcanization is carried out in the presence of amine salts of polyhydrosulphuric acids which contain 7 or 8 sulphur atoms to 2 mols of the amine.

The compounds used according to the invention act as excellent vulcanizing agents and powerful accelerators, both in natural and synthetic rubber. In addition, the mechanical properties of the vulcanizates and especially the ageing thereof are substantially improved.

The polyhydrosulphuric acid amino salts used for the process of the invention can be represented by the following formula:

in which A represents the amines and in which formulae, R R and R can each represent an alkyl, cycloalkyl or aralkyl radical which radicals may be branched and substituted e. g. by hydroxy groups, and R and R each can represent an alkyl radical, prefera'bly a lower alkyl radical containing up to 5 carbon atoms, and R R can represent additionally a hydrogen atom. The radicals R and R can furthermore form a heterocyclic ring with the nitrogen atom, and this ring may contain other heterocyclic atoms, such as N, O or S atoms. 11 is 7 or 8.

The polyhydrosulphuric acid salts of amines can for example be obtained in a simple manner if sulphur is added to an alcoholic solution of the amine and hydrogen sulphide is introduced into the suspension which is formed, preferably while stirring.

hlllfifi Fatented Nov. 19., 1%53 Primary, secondary and also tertiary amines are suitable for employment in the process according to the present invention, and especially those which have a dissociation constant at 25 C. which is greater than 10- Examples of suitable amines are diethylamine, monoe-thanolarnine, di-N-butylamine, tert. butylamine, triethylamine, cyclohexylamine, dicyclohexylamine, N-methylcyclohexylamine, benzylamine, pyrrolidine, -N-methyl pyrrolidine, piperidine, morpholine and hexarnethylene imine.

Low molecular weight aliphatic alcohols, such as methyl alcohol, ethyl alcohol, isopropyl alcohol and tertiary butanol are particularly preferred alcohols to employ as solvents in the process according to the present invention. The quantity of solvent employed may be just sufi'icient to maintain a suspension of the reaction mixture which is just capable of being stirred. The process according to the invention is preferably carried out at room temperature or at a temperature slightly above room temperature, for example at a temperature of from 10 C. to 50 C. and preferably at a temperature of from 15 C. to 35 C. However, higher or lower temperatures can also be used. The reaction time depends on the quantity of hydrogen sulphide introduced per unit of time. The hydrogen sulphide is absorbed very quickly, so that the reaction is complete in a relatively short time. The sulphur is preferably added in a stoichiometric amount.

During the introduction of the hydrogen sulphide, the elementary sulphur enters into solution. The reaction mixture is preferably stirred during the introduction of the hydrogen sulphide. The reaction is complete when no further absorption of hydrogen sulphide takes place. The amine salts of polyhydrosulphuric acids are predominantly unitary solid compounds.

They have a yellow to orange-red colour and frequently separate out during the introduction of the hydrogen sulphide. In some cases, the reaction products are viscous oils, which frequently crystallize after standing for some time. The amine salts of polyhydrosulphuric acids are slowly split into their initial components by the action of Water, and more quickly by means of acids.

The sulphur and amine contents of the amine salts can be determined by elementary analysis and/or by amine titration and are generally in the region of 7 or 8 sulphur atoms per 2 mols of amine. This sulphur content is independent of the quantity of sulphur introduced, but naturally the yield falls when the added quantity of sulphur is too low.

The process according to the present invention is illustrated by the following reaction diagram, in which diisopropylamine is the amine component.

Several processes have already been described in the literature for the production of some amine salts of polyhydrosulphunic acids, but which have several disadvan tages when compared with the process of the instant invention. Thus, it has been proposed to react an amine in alcoholic solution with an alkali metal polysulphide solution, to introduce air and hydrogen sulphide into an alcoholic solution of an amine, and also to react an aqueous solution of an amine salt with an ammonium or alkali metal polysulphide. These processes produce amine salts of polyhydrosulphuric acids, but only in admixture with other compounds from which it is difficult to separate the desired amine salts. By employing the aforementioned prior art processes a poor yield of amine salt is obtained in many cases. According to another process amine salts of polyhydrosulphuric acids are obtained by introducing hydrogen sulphide into a homogeneous solution of amine and sulphur. In this process dipole-free liquids, particularly benzene, are employed as solvents. This latter process, however, cannot be employed for the production of amine salts of polyhydrosulphuric acids on a commercial scale because the yield per unit volume and per unit time is too low.

In contract thereto are the polyhydrosulphuric acid amine salts according to the above mentioned preferred process for the production of these compounds obtained in a practically quantitative yield and extremely pure.

A number of the compounds suitable for the process of the invention are set out in the following table.

dimethyl butadiene, isoprene and their homologues, or the copolymers of these compounds with polymerisable vinyl compounds, such as styrene, a-methyl styrene and their substitution products, acrylonitrile, methacrylonitrile, acrylic and methacrylic acid esters and similar compounds or copolymers which are obtained from isoolefines, such as for example iso-butylene and its homologues, with relatively small quantities of conjugated diolefines. Furthermore, polymers of chlorobut-adiene and its copolymers with monoolefines or diolefines or other polymerisable vinyl compounds are suitable.

EXAMPLE 1 A test mixture which contains 20.0 parts by weight of siliceous chalk 10.0 parts by weight of titanium dioxide TABLE 1 M.P., C. Properties [(C2 5)2N ]2.H2Ss 89-91 Orange crystals.

(2) [(CH3)3CNH2]2-H2S7 137-140 D0- (3) [(CHmCHNHZhH S 137-139 Yellow-orange crystals.

(4) [OH3(CH) NH2]2.H2S -72 Orange crystals.

( [(CzHs) aN]2.H2S1 101 Yell0w-orange crystals.

(6) [CHa(CH:) NH2]2.H2S7 100-102 D0.

(7) H NH2].H2S3 145-147 D0.

(8) H )NH].H;S7 147 Orange-red crystals.

(9) [@Nnomlmsi Red viscous liquid.

1o) [@momnlms, 73 Orange-red crystals.

(l1) H NH .H2S7 Red viscous liquid.

(12) H N H].HS1 -82 Yell0w-0range crystals.

(13) I: H NHJ-HZST Red viscous liquid.

m (14) 0 NH .H2S7 Yellow'orange crystals.

\ /C =NH H2S7 115 D0.

The polyhydrosulphuric acid salts used according to the invention are added to the rubber mixture, generally in a quantity of from 0.5 to 15% by weight, preferably 1 to 10% by weight, calculated on the rubber content present; if necessary, in addition to the usual additives, such as for example accelerators, age-resistors or fillers. The polyhydrosulphuric acid amine salts can be used in a pure form, or they may be blended with inactive or active fillers or as adsorbates on siliceous chalk, clay, silicic acids or silicates, such as zeolites, or mixed with carbon blacks. Occasionally, it is advantageous to produce the polyhydrosulphuric acid amine salts in the presence of such fillers. Furthermore, the salts can be used in the form of a paste, employing mineral oils or rubber plasticisers, or dispersed in suitable hard paraffins and then show good dispersibility and stability.

As well as natural rubber, it is also possible to use synthetic, rubber-like polymers whch can be vulcanized with sulphur and which are obtained for example by polymerisation of conjugated diolefines, such as butadiene,

10.0 parts by weight of zinc oxide 1.0 part by weight of stearic acid 1.0 part by weight of tetramethyl thiuram disulphide A. 1.0% of sulphur B. 1.7% of the piperidine salt of heptahydrosulphuric acid 12) C. 1.9% of the hexamethylenimine salt of heptahydrosulphunic acid 13) D. 1.9% of the cyclohexylamine salt of octahydrosulphuric acid (7) E. 2.6% of the dicyclohexylarnine salt of heptahydrosulphuric acid (8) F. 1.7% of the diethylamine salt of octahydrosulphuric acid 1) G. 1.9% of trlethylamine salt of heptahydrosulphuric acid (5) H. 1.6% of tert. butylarnine salt of heptahydrosulphuric acid (2) I. 2.7% of the n-dodecylamine salt of heptahydrosulphuric acid (6) K. 1.6% of the ethanolamine salt of heptahydrosulphun'c acid L. 2.0% of the 'benzylamine salt of heptahydrosulphuric acid M. 2.9% of the N,N'-diphenylguanidine salt of heptahydrosulphuric acid ('15) After vulcanization in a press, the Schopper ring test (for information see Memmler, Handbuch Kautschuk Wiss. American Edition, New York, 1934, page 52 9 et seq.) gives the following comparison values for the modulus at 300% elongation (the first number in each case) and the tensile strength in kg./cm. (the second number in each case):

6 as powerful activators. They render possible a vulcanization with a shortened heating time and reduced tempera ture, and furthermore produce vulcanizates with excellent ageing properties. The amine salts of the polyhydrosulphuric acids can be obtained by the following procedure:

(a) Dicyclohexylamine Salt of Heptahydrosulphzm'c Acid 362 g. (2 mols) of dicyclohexylarnine were dissolved in 900 cc. of methanol and 19 2 g. (6 gram atoms) of sulphur were added to the resulting solution in powder form. Hydrogen sulphide was then introduced into the suspension with stirring at 20-40 C. until no more hydrogen sulphide was absorbed (4 hours), the sulphur gradually entering into solution. The mixture became red in colour. Orange-red crystals precipitated out from the reaction miXture and were filtered with suction, Washed with ether and dried in air. Yield 540 g.=92% of the theoretical, M.P. 147 C.

By reacting equivalent quantities of amine and sulphur in homogeneous solution with benzene as solvent, at least 12.5 litres of benzene and substantially longer reaction times are required.

TABLE Vulcanizer A B C D E F G H I K L M The mixtures containing the compounds used accord- 35 C H N S (molecular weight 588)Calculated: C, ing to the invention show a strong vulcanization at a 49.0%; H, 8.1%; N, 4. 8% S, 38.1%. Found: C, 49.1%; low vulcanization temperature. H, 8.2%; N, 4.6%; S, 37.1%. Calculated amine con- On examining the ageing after 14 days in the oxygen tent: 61.5%. Found by titration: 60.1%. bomb (21 atm. gauge oxygen/ 60 C.) the following ten- By using other amines, it is possible in like manner to sile strengths (first number) (in kg./cm. were obtained obtain the amine salts of the polyhydrosulphuric acids and, after 28 days, under the same conditions the following percentage increases in Weight (calculated on the rubber content (second number) indicated below. Thereby it is possible to use other alcohols such as ethanol, isopropanol or tert.-butanol, also in varying amounts e.g. 400-500 cc. instead of the afore- TABLE Vuleanizer A B C D E F G H I K L M In the present case, therefore, the compounds accordmentioned '900 cc. The mixture obtained should be only mg to the invention are effective as sulphur donors and sintable.

Reaction Product Calc. Found Mols Amine Summation Amine Gram- M .P. formula atoms Yield Colour C. N S N S sulphur Diethylarnine 7 86 6. 9 63. 3 6. 9 63. 5 CgHaiNgSg Triethylamine 6 6. 5 52. 3 6. 5 50. 8 C12H32N2S7 Di-n-buty1amine 6 91 5. 8 46. 3 5. 9 47. 6 CmHmNzS Diisopr0pylamine 6 96 6. 5 52. 4 6. 7 52. 8 C12H32N2S7 n-D0decy1amine 6 9O 4. 7 37. 6 4. 7 38. 0 C24H55Ng$7 Cyclohexylamine" 7 82 6. 1 55. 9 6. 1 55. 5 OflHisNzSg Pyrro1idine 6 93 7. 8 60. 7 7. 6 60. 6 CBHZDN2S7 Piperidine 6 91 7. l 56. 6 7. 0 55. 0 C1nH74NzS7 Hexamethy1en1m1ne 6 92 6. 6 52. 8 6. 4 53. 0 C qHnN1S1 Morpholine 6 87 7.0 56. 0 6. 8 56. 8 C8Hz0OgNgS N-Dimethylcyclohexylam 6 5. 8 46. 5 5. 4 46. l C10H30Nz5 N,N-Diphenylguanidine 6 71 12. 35. 2 12. 48 36. 2 CMH2BNQS7 Ethsmnlqminp 6 93 8. 1 64. 5 8. 1 66. 7 c HmozNzs-r Benzylamine 6 83 6. 31 51. 0 6. 1 53. 2 C14H20Nzs7 N,N-Di-otolylguanidine 6 80 11. 9 31. 9 11. 9 32. 8 C3OHMN5S1 NOTE.-y=yell0w. y.0.=yel1ow-orange. o.r.=orange-red. r. 0il=red oil.

7 EXAMPLE 2 A test mixture which contains 100.0 parts by weight smoked sheets 30.0 parts by weight EPC-black 10.0 parts by Weight MAP-black 6.0 parts by weight zinc oxide 3.0 parts by weight stearic acid is divided into three equal parts and one part each is mixed with one of the following additives. The quantities relate to the rubber content. No sulphur and no vulcanization accelerator is added.

A. 4.9% by weight of the di-isopropylamine salt of heptahydrosulphuric acid B. 3.7% by weight of piperidine of heptahydrosulphuric acid C. 6.8% by weight of the di-o-tolylguanidine salt of heptahydrosulphuric acid After vulcanization in the press the Schopper ring test gives the following comparison values (the first number gives the modulus at 300% elongation, and the second number gives the tensile strength in kg./crn.

Vulcanizer A B C 1.0 atm: 25' 15-90 23-135 14-100 2.5 atrn:

EXAMPLE 3 A test mixture which contains 20.0 parts by weight of SRF carbon black, 4.0 parts by weight of magnesia and 15 .0 parts by weight of zinc oxide per 100.0 parts by weight of chlorobutadiene polymer is divided into four equal parts and each part has added thereto one of the following additives, based on the polymer content:

A. 1.65% by weight of the dicyclohexylamine salt of heptahydrosulphuric acid,

B. 1.65 by weight of the cyclohexylamine salt of octahydrosulphuric acid,

C. 1.65% by weight of the dodecylamine salt of heptahydrosulphuric acid,

D. without additive (for comparison purposes).

After vulcanization in the press, the Schopper ring test carried out under the same conditions as in Example 1, produces the following comparison values (the first number gives the modulus at 300% elongation and the second number gives the tensile strength in kg./cm.

Vulcanizer, A B C D EXAMPLE 4 A test mixture which contains 100.0 parts by weight butadiene-styrol-copolymer 44.0 parts by weight HAP-black 5.0 parts by weight zinc oxide .0 parts by weight mineral oil softener 1.5 parts by weight stearic acid 0.8 part by weight parafiin wax 1.0 part by weight wool fat 1.6 parts by weight benzothiazy1-2-dicyclohexylsulfenamide 1.0 part by weight phenyl-a-naphthylamine is divided into seven equal parts and one part each is mixed with one of the following additives, respectively additive mixtures. The quantities relate to the rubber content, whereby the compounds used according to the invention are supplied in a higher dosage according to their higher molecular weight (compared with the sulphur):

A. 2.0% by weight sulphur B. 2.6% by weight of the dicylohexylamine salt of heptahydrosulphuric acid (8), 1.0% by Weight sulphur C. 5 2% by weight of the dicyclohexylamine salt of heptahydrosulphuric acid (8) D. 1.9% by weight of the diisopropylamine of heptahydrosulphuric acid (3), 1.0% by weight of sulphur E. 3.8% by weight of the diisopropylamine salt of heptahydrosulphuric acid (3) F. 1.7% by weight of the piperidine salt of heptahydrosulphuric acid (12), and 1% by weight of sulphur G. 3.4% by weight of the piperidine salt of heptahydrosulphuric acid (12) After vulcanization in the press the Schopper ring test gives the following comparison values, whereby the first number indicates the modulus at 300% elongation and the second number the tensile strength in kg./cm.

Vulcanizer -5 A B O D E F G 0 5 atm:

EXAMPLE 5 A test mixture which contains 100.0 parts by weight of butadiene-acrylonitrile-copolymer 44.0 parts by weight of HAP-black 5.0 parts by weight of zinc oxide 5 .0 parts by weight of methylene-bis-thioglycolic acid-butyl ester 1.5 parts by weight of stearic acid 08 part by weight of paraifin wax 1.6 parts by weight of benzothiazyl-2-dicyclohexylsulfenamide 1.0 part by weight of phenyl-u-naphthylamine is divided into seven equal parts and one part each is mixed with one of the following additives, respectively with one of the additive mixtures. The quantities relate to the rubber content. The compounds according to the invention are added in such amount that the sulphur content of the additives, respectively additive mixtures corresponds to those cases in which only sulphur is added:

After vulcanization in the press the Schopper ring test gives the following comparison values (the first number 9 in each case gives the modulus at 300% elongation and the second number the tensile strength in kg./cm.

B O D E F Vulcanizer---.-

EXAMPLE 6 Into this paste mixture there are dived test tubes with a diameter of 30 mm. The films formed on the test tubes are dried and vulcanized at about 100 C. The lower part of this film is removed from the test tubes, and the remaining pipe like film is rolled from the lower part to the upper part, thus forming a rubber ring having a diameter of about 1 mm. With this rubber ring the Schopper ring test is carried through whereby the following com- .parison values were obtained. (The numbers indicate the tensile strength in kg./cm.

Vulcanizer A i B I C J D E F G 1 0 EXAMPLE 7 A test mixture containing 100.0 parts by weight of isobutylene-isoprene-polymer 40.0 parts by weight of HAP-black 5.0 parts by weight of zinc oxide 2.0 parts by Weight of stearic acid is mixed in a kneader at a temperature of 175-180 C., thereafter it is divided into 3 equal parts, and one part each is mixed on a roller with one of the following additives (the quantities relate to the polymer content):

A. 3.2% of the ethanolamine salt of heptahydrosulphuric acid, 1% thiuramdisulfide and 0.5% mercaptobenzothiazole, each part separately being added,

B. 3.9% of the diisopropylamine salt of heptahydrosulphuric acid and 1% thiuramdisulfide, and 0.5 mercaptobenzothiazole, separately being added,

C. 3.5% of the piperidine salt of heptahydrosulphuric acid, and 1.0% thiuramdisulfide and 0.5% mercaptobenzothiazole, separately being added.

After vulcanization in a press the Schopper ring test gives the following comparison values (the first number give the modulus at 300% elongation, the second number in each case indicates the tensile strength in kg./cm.

Vulcanizcr A B C 10 atm: 10 13-125 15-130 17-130 4.0 atm:

We claim:

1. In a process for the vulcanization of a member selected from the group consisting of natural rubber and a sulfur-vulcanizable diene rubber, the improvement which comprises employing as vulcanization agents amine salts of polyhydrosulfuric acids which contain a member selected from the group consisting of 7 and 8 sulfur atoms to 2 mols of the amine.

2. The process of claim 1 wherein said amine salt is the cyclohexylamine salt of octahydrosulfuric acid.

3. The process of claim 1 wherein the amine salt is the dicyclohexylamine salt of heptahydrosulfuric acid.

4. The process of claim 1 wherein the amine salt is the diisopropylamine salt of heptahydrosulfuric acid.

Krebs et al.: Z. ahorg. u. allgem. Chem. 275, pages 147-51 (1954).

Chem. Abstracts, 49, pages 15, 726, November 25, 1955. 

1. IN A PROCESS FOR THE VULCANIZATION OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF NATURAL RUBBER AND A SULFUR-VULCANIZABLE DIENE RUBBER, THE IMPROVEMENT WHICH COMPRISES EMPLOYING AS VULCANIZATION AGENTS AMINE SALTS OF POLYHYDROSULFURIC ACIDS WHICH CONTAIN A MEMBER SELECTED FROM THE GROUP CONSISTING OF 7 AND 8 SULFUR ATOMS TO 2 MOLS OF THE AMINE. 